How I Met His Sister
by Tainted Ideals
Summary: Fiolee. Chapter book. Fionna is staying over at Marshall's for the weekend. She was hoping to sleep over, until she meets Marceline, Marshall's older sister! How will just two days, Friday night, Saturday morning, Saturday night, and Sunday Morning, result for the adventuress and her evil in-law? Rated T for language.
1. Meet Marceline! My evil sis!

**Fiolee presents: How I met your sister.**

**Chapter one: Meet Marceline! My evil sis!**

**Okay, who wants me to do a fiolee and link the kingdom of Ooo to the kingdom of Aaa?**

**Massive Fiolee shipper crowd: WE DO!**

***sweatdrop* Okay, on to the story.**

* * *

Fionna knocked on the door."Heya, baby," greeted her boyfriend, Marshall Lee, at his front porch.

Fionna blushed. Five months and you'd think she's used to him calling her "baby".

"Hiya, Marshall. Ready for our two-person sleepover?" she asked eagerly as she stepped inside his living room and set her stuff on his couch.

Marshall stared at her. Whenever it was about "just the two of them alone" moments, Fionna dressed a tad girlier than her usual self, a little more strict, and even wore outfits she would never use anywhere else. Like today, or tonight, she no longer wore her bunny ear hat, tied her hair in a long ponytail that reached her low waist, wore a black t-shirt, and knee-length green khakis.

"There's just one problem," he admitted, guiltily scratching the back of his head.

The eager blonde frowned and held her hips. "What now? Last week you said that you didn't have extra butter for the popcorn was a problem. And yesterday, you told me that a relative staying at your place was a problem." Her eyes widened in realization. "No...Don't tell me..."

He stuck his hands in his pockets and guiltily walked around the room. "Yeah. It's my sister. She's visiting for the weekend, and I don't know if you'll have any problems with her."

"Marshall Lee, we've been together for five months and you never found the decency to tell me even once that you had a sister?" she fumed crossing her arms. See? This is the strict that no one ever expected from her. Except, of course, for her boyfriend.

"I swear I tried! But she's too..." he scratched his bite marks thoughtfully. "Extreme? I don't know, but the thing is that she's here and-"

"Hey little bro!" A girl vampire said coming from the kitchen. She sported a long wool sweater dress with black and white stripes, black fish net tights, long loose black hair, and black heeled leather boots. "Ooh, is this the girl you keep rambling on about?" Marceline asked her brother, examining Fionna from head to toe.

"I guess I am," Fionna grinned at meeting Marshall's older sister. She bowed. "Fionna the human, Marshall's girlfriend."

"Marceline Abadeer, Vampire Queen, and Marshall's _older _sister," the vampiress returned in the same way. "You're human right? Do you by any chance happen to know Finn the Human?" she asked.

Fionna nodded. "Yes! He's my cousin," she replied.

Marceline then rolled in the air laughing. "Wow, I thought I'd never meet that goodie-two-shoes' sis! So, how's life been, my little in-law with my little brother?"

Marshall frowned. "I told you to stop calling me little! We're almost the same age, Marcy!"

Marceline just smirked. "Pffft...whatever. How old are you again, bro? Nine hundred ninety-five? Oh, oh, do I hear one thousand plus? Of course I don't. I'm the one with one thousand plus years of life. Thus, making you younger."

"So? At least I'm not daddy's little princess!" Marshall returned.

"Oh yeah?! And are YOU, mister, dad's favorite? Neither am I!" His sister spat.

Fionna sweatdropped and took her bag. "I'm gonna go unpack upstairs..."

Leaving up the stairs, tuning out the bickering siblings, the young human girl headed towards Marshall Lee's bedroom. Closing the door behind her, she addressed herself towards her assigned dresser and started unpacking clothes from her backpack into the drawers.

Lately, Fionna found herself "sleeping over" at Marshall's three times a week. So he decided to build a dresser at his room for her clothes, and the clothes she only used around him were in these drawers. And anyway, staying behind at the tree fort was lonely, because Cake and Monochromicorn have been a little...busy. And now Cake's got to take care of five kittens of her own, so yeah, nobody's at the tree house anymore, not even Beemo.

Suddenly, Fionna heard the door slam shut behind her. "Ugh, I can't believe him!" Marceline exclaimed. Seeing Fionna and their surroundings, Marceline began floating in the air.

"Oh, sorry Fionna. I just wanted to shut myself inside a random room, but instead of my guest room, I guess I wound up in my brother's crib," she apologized.

"Oh, no worries. All good. What happened out there? I came back up after the daddy's favorite bit," Fionna replied.

Marceline floated down to the floor and sighed, sitting down casually. "It's nothing. Just the normal six p. m. bickering. By the way, why are _you _in my brother's crib. Sleeping over for the night?"

Fionna blushed as she stuffed the last set of clothes and her backpack in the drawer. Sitting on the edge of the bed, she brushed a strand of her hair out of her face. "Y-Yeah. I was just unpacking my clothes in the dresser he made for me. I've slept over for the past couple weeks."

Marceline nodded. After two seconds of silence her lips curved into a smirk. "And by any chance," the vampiress said sitting up on the bed next to Fionna, "Have you done..._it_?"

Fionna frowned in confusion. "It what?"

Marceline groaned. "You know..._it. _How do I put it? You know...if Cake ever taught you the "tiers" of a relationship...'do not do tier fifteen' sound familiar?"

Fionna shook her head. "Yeah, I remember that explanation. But all she said was to never do tier fifteen, though she never said what it was..."

Marceline crossed her arms. Damn, this girl is _hopeless. _It's only luck that ever brought those two together.

"Well," Marceline said. "Have you kissed in bed?"

Fionna blushed. "No."

"Well, you should," Marceline continued, her smirk enlarging. Time to gross this girl out of her mind. "I've done that with my ex, it's...fun, I guess, when you're alone . But you're not moving in yet?"

Fionna shook her head once again. "Nope, I'm not ready for that. Someone needs to keep the treehouse's maintenance and Cake's taking care of her kittens and...I just don't feel ready to take such a big step. And plus, Cake would be devastated. You know how sensitive she is about this whole chizz? It's just luck that she lets me off the hook three nights a week!"

Marceline mentally face palmed. Damn, this girl is just as goody-two-shoes as Finn. To brainwash her into the grossest truths of a relationship is pointless. "I understand. Come on, I think Marshy-boy's cooled off by now," Marceline said floating towards the door.

Turning the doorknob, and realizing Fionna wasn't coming, she said, "So? Come on!"

That snapped her out of her thoughts. "Coming!"

As they walked down the stairs, Fionna wondered why Marshall hated his sister so much. To her, Marceline was the raddest friend ever.

* * *

**So, how do you like Marcy's evil-ness? Keep following this story, 'cause it's kind of short!**


	2. Talk to me

**Welcome to chapter 2 of "How I Met His Sister!"**

**This is actually my first improv, so basically, I'm just making it up as I go along.**

* * *

_As they walked down the stairs, Fionna wondered why Marshall hated his sister so much. To her, Marceline was the raddest friend ever._

* * *

At least, that's what Marcy wanted her to think. Sure, everyone knows Fionna is gullible, but not so gullible, but Marceline loved to play mind games and traps to people she's barely met. Like Finn. And if Fi is just as gullible as her cousin look-alike, all Marcy had to do was play "little-old-herself". The old herself Marshall hated ever so much.

As the girls went down the stairs, Marshall appeared from the kitchen, blithe as ever, as if he'd never bickered with his sister.

"Hey, girls," he said sliding on an oven mit, "Fionna, would you mind helping me in the kitchen?" She nodded and went to help her boyfriend out.

As Fionna closed the kitchen door behind them, she strapped on a red apron. She wondered why he hadn't sucked the red out of it yet.

Shaking it off, the blond unconsciously began stirring a batter. After a few seconds, she dipped a finger in the batter, tasted it, and asked Marshall Lee, "Dude, what the hell are we making?"

The young vampire turned around from his post in front of the fridge and set some food on the counter. "Pizza. Cheese pizza. Because I can actually **eat **the red sauce without the temptation of drinking the red out of any pepperoni." Marshall chuckled and instructed Fionna to keep stirring that batter while he grated some cheese next to her.

In the silence that filled the two, Fionna reflected over what her in-law had told her.

_"Well, have you ever kissed while you were in bed?" _Those words circled the poor blonde's mind. She felt flustered. Too flustered for a fourteen-year-old. Sure, it was certain that those thoughts were filtered to stay out of her brain, but the more she went over it inside her head, the more right they sounded. More...erhm, desireable.

Realizing how deep in thought she was, Marshall looked concerned at his girlfriend when she was suddenly starign into space and stirring the batter in lightning speed.

"Fionna?" he spoke softly. She still continued her actions. "Fionna..." he said a little louder. No result. "Fionna!" Marshall Lee held her wrists, making her blink, noticing her behavior. If there's one thing Marshall knows about Fionna that not many besides Cake know, is that she tends to do whatever she is doing faster than what is humanly possible, if she's thinking of something that makes her nervous.

Fionna's face flushed as she jumped in a startled state. Her eyes darted between the bowl of soupy batter, the position of her hands, and how they'd been locked by the black head's hands, analyzing what just happened.

"Are you okay? You had me concerned when you began stirring like flash," he said.

"Is everything okay in there?" Marceline yelled from the living room.

"Yeah, sis! Fionna just dropped some flour, that's all!" Marshall responded. He then turned back to the stunned human girl. "Talk to me," he whispered. Then holding her in a close embrace by her waist, he repeated the same three words. "Talk to me."

Fionna blushed and politely pulled away, not really obtaining to pull away, because he just held her tighter and closer. Giving up, she sighed and rested her head on his chest.

"I'm sorry, I just have this thought in my mind that I can't send to the vault," she admitted in a low voice.

The vampire king nodded. "Okay, what did my sister tell you?"

The blonde girl's head sprang up to face his. This really and overly took her by surprise more than what had happened barely moments ago. When she'd explained precisely what happened, and that she was even more flustered that not knowing what _it _was, Marshall Lee held her even closer and stared into her blue eyes, almost **(****ALMOST) **looking straight through her innocent soul.

"Tell me," he said landing a soft kiss on her lips, "Do you want to me to kiss you differently tonight?"

"Erhm...I don't know. I guess. But it feels a little...serious. Won't Cake turn all ballistic? You know Monochromicorn sees everything at her's or PG's command," Fionna replied speaking like a chipmunk. It was her responsible, goody-two-shoes self, that kept her from adventuring that much anymore. And speaking like a chipmunk was a clear sign that she was nervous and uncertain about the conflicting decision. Yes or no?

She sighed. "Fine, I give in. Just, not too much. I've heard about your history," she said. She was going to regret it, she just knew it.

"Good," he replied smirking. "Tonight. Just before you go to sleep."

* * *

Marceline smirked to herself as she eavesdropped the conversation from the couch. Bat ears, baby. They do wonders.

So...she finally managed to break the girl through the power of a couple of words. Phase one, complete. Now, to wait until tomorrow morning. Let's see if she can break her further without going overboard.

Oh, how excited she felt. Not because they were going to watch Heat Signature Uncut, or because she already had tomorrow planned out, but rather how fast her little baby brother was growing up.

And she thought he was a wuss.

* * *

**So, how did you like this chappy? I'm sorry if my stories already bore you ever so much and handily. This is my first improv, so I can't promise updating soon. And I have writer's block, which is why I haven't continued Pokemon High School.**

**But fear not! I shall have the next chapter ready just in time before my last exams. So don't expect much from me because my final grade exams are in two weeks, and I graduate from fourth grade shortly after!**

**Wish me the best of luck!**


	3. Hick WHAT now?

**Chapter 3: Hick WHAT now?**

**Umm...Just skipping to the next day, Saturday morning, and eh... I am so embarrassed at what my perverted self is FORCING me to do.**

**Perv me: Hey, you said you worked with your engineering imagination, so I gave your engine some fuel. Now, KEEP TYPING!**

**Regular me: *sigh* Fine. Please enjoy this...or not.**

**I feel like such a pervert and I haven't even begun to type...**

* * *

_Fionna's POV_

Ugh...why does my head hurt so much? And what's this pain in my neck?

I opened my eyes to Marshal's dark bedroom. Oh riiiiiiiiiiiiiiiiiiiiiiight. I'm spending the weekend, aren't I? Eh...

I looked around. The door was closed, the bathroom was open, and Marshal was nowhere in sight. So he must've woken up early.

I grunted and sat upright. Let's see, what the glomp happened last night? Because my head's spinning, and I'm not sure I understand why.

...

...

I flushed darkly recalling what happened. We watched Heat Signature Uncut, then when Marshal and I came to sleep...he began kissing me but, I don't know...like, a long time. And then he began kissing my neck, and he left some sort of bruise...

I felt the my neck with my hand and sure enough, the bruise was still there, making me blush more.

I quickly tied my messy loose hair back into a ponytail and ran to the bathroom.

I looked at my reflection in the mirror and saw that the bruise was a little...noticeable.

"Slam-a-cow!" I exclaimed. "Cake's gonna be raging if she sees this!"

"See what?" a voice asked behind me.

I spun around and sighed when I noticed it was just Marceline.

"Oh, Marceline, it's only you," I sighed. "I thought it might've been Cake who could've been stalking me. Hey, what's this bruise? Do you know when it might heal?"

Marceline floated towards me. She dressed differently from last night. Today, she wore a black sweater with a single neon green stripe, dark blue-jeans, black flip-flops, and her hair was held in a low ponytail, where two side bangs hung loose.

"Hmm..." Marceline said inspecting me. "Did my brother by any chance...kiss your neck...last night?"

I bashfully nodded, a fully red flush taking over my cheeks. "Yes, a little."

Then, Marceline rolled around, in the air, holding her stomach, LAUGHING. "Hahahahahahahahahahaha! I can't believe this! My brother gave you a hickey! Hahahahahahahaha!" she yelled.

"A hick **what** now?" I asked, not following.

Marceline, still laughing, managed to wheeze through uncontrollable laughing pauses, to catch her breath. "A hi—a hi—, a HICKEY! Hahahahahah!"

I began to laugh nervously. "Eh, heheh...what's so funny?"

Then she wiped a stray tear from her face and tried her best to regain composure...and her breath. After such a mad laughing fit, I would be out of breath too.

"I'm sorry," Marceline said, "It's just that it's so _small_! He couldn't _possibly_ be proud of a hickey that small."

"Umm, Marceline?" I asked.

"Yeah?"

"What's a hickey?"

Then the enormous grin on her face, slowly faded. And it slapped away from her face with a face-palm.

"Dang, what else does Cake **not **teach you?" she grumbled. Then looking back at me, she crossed her arms. "Well, a hickey is that bruise you got right there," she explained pointing at the blue-ish purple point on my neck, "And it's the result of kissing your neck while sucking it a little."

I made a disgusted face. "Sucking?" She nodded. "Ugh, gross. Does this mean he left drool on me?"

Marceline shook her head. "No, thank globness gracious, no, he didn't leave drool. It should be gone by tomorrow. And don't worry, I've seen bigger hickeys. Believe me."

I managed a slight chuckle. At this point I'm not sure who to believe anymore.

* * *

I went down to the kitchen once I'd changed. I wore a baby blue, cotton turtleneck shirt, because it was chilly, and because, if Cake decides to arrive unexpectedly, she won't see this. I wore a ponytail on my hair again and black jeans with black sneakers.

Then, letting myself into the kitchen, Marceline on the couch, I tapped Marshall's shoulder.

He turned around and smiled. "Hey, did you have a good rest?"

I crossed my arms and stretched the collar of my shirt. "Mind telling me what this **is?!**"

Then he looked at the "hickey" and drew a sharp breath. "Ooh...nice isn't it?" Then he turned back to making pancakes.

I switched off the stove and turned him around. "Dude! I thought you were only kissing me! Not sucking!" Wow, that sounded better in my head.

"I was! It's a type of kissing," he calmly said.

"You think this is normal?" I seethed. "I know it's gonna heal before I get back to the Tree Fort, but it's...weird! Why did you do that?"

"Hey, cut me some slack," Marshal said, "I _suggested _the idea, and you firmly _agreed. _See where I'm going? You didn't HAVE to accept! So now, you suffer a small consequence. Now, would you mind setting the table?"

I crossed my arms and pouted. "Fine."

Dang, sometimes I hate the truth of things.

Even less when it comes from him.

* * *

After breakfast, Marshall and I went into the woods and see what we could do or beat up. I still wore my turtleneck, and jeans, so we decided going to the Ice Kingdom, or at least the outskirts of, while Marceline decided to stay behind and watch T.V.

As I flew on his back, in the giant-bat form, I began to speak softly.

"I'm sorry," I whispered.

"Wazz dat?" he asked. I chuckled. As if he couldn't hear everything in his giant amplified bat ears.

"I'm sorry," I said louder. "I'm sorry for the outburst from this morning, but I really was mad."

Marshall chortled. "You got that right."

I leaned closer to his fur and hugged him. "Forgive me?"

"Anytime."

The he grabbed my arms and transformed back to normal, lowering us to safer ground.

Once he let me go, I began to regret not bringing my hat.

"Gob, why didn't I bring the hat when I had time?" I complained.

"Ya mean, this hat?" Marshall said holding out my hat from his pocket.

"Yay!" I giggled, took it, and pecked his cheek. "Thanks."

Then I whipped out my sword from my backpack and walked ahead.

"Hey," Marshal said, "What do you think Ice Queen's doing right now?"

I looked at the mountain in thought. "I dunno. Wanna check it out?" I asked

"Race you there!" he yelled.

"Hey!" I giggled. This much running might actually be good for me. And the cold might help the "hickey" disappear.

* * *

**So, did you like? Did you hate? Was it funny? I don't feel perverted anymore, yay!**

**Perv me: Darn.**

**So, review follow favorite or not! **

**Goodbye !**


	4. Good Cops, Bad Cops and One-sidedness

**Chapter 4: Good Cops, Bad Cops and One-sidedness**

**Okay, I just got a totally amazing and exciting idea to a certain reviewer, kevinkev18. Welcome to a new chapter with a small yet significant hint of one-sided Finnceline!**

**Enjoy. Or not. If you don't enjoy, you can sit there and expect me to care.**

* * *

_Fionna's POV_

Today, while Marshal was out for food in the morning (today's Sunday morn, just a few while before I leave) I decided to go solo on good cop/bad cop on Marceline.

You see, yesterday, I began to notice a little detail. She induced _me _into walking straight into that "hickey trap". And even though it's already healed, Marceline seemed to somehow play tricks on me as if she _knew _me. And believe it or not, that is suspicious for most people.

As she watched T.V., I decided to make my first appearance as good cop.

I walked in front of the T.V. and said,

"Hey Marcy, can I have a talk with you?"

She groaned loudly and rudely. "Yeah, whatever. Can you just not block the screen? I am watching it, you know."

I stepped away from the television and grabbed a chair. I sat across from her and switched instantly to bad cop. Sort of.

"Why the tricks?" I asked. That caught her attention, because she switched off the T.V. and looked straight at me.

"Come again?" she asked.

"Don't play with me Marceline," I said. As of then, I had no absolute control over my words. "You were playing mind tricks with me to try to mess me up, which you kind of did, but all the time, it seemed like you already had done this kind of games before. Like you knew me."

I paused to let out a slight smirk. "Is my cousin that familiar? Or are you also together?"

Then, out in the open field that was the living room, anyone could see, that Marceline's cheeks turned **red**. Yes, she was blushing.

"Uh-n-no! N-no, we're not. And n-no, your cousin is not _that _familiar. It's ju-just that I used to hang out with him a lot and...Oh, don't mention any of this to my brother, would ya?" she whined. "Sure, he's my brother, but the bad thing is he's my _brother_! Any part of this conversation could be used against me."

I raised and eyebrow. For a response, that was longer than I expected. "Deal," I said, "Your secret's safe with me. But answer my question, why did you play all those little tricks?"

Marceline sighed. "You know, you're the only girlfriend my brother's had that hasn't broken his heart...yet. And well, he never got that far along Relationship Lane with Ashley, his ex-girlfriend. All I wanted was to...I dunno, see him grow up. He is 5 years younger than me."

I nodded and dropped the whole good cop/bad cop scheme.

"We cool?" Marceline asked.

I put a finger on my chin. "Hmm...I guess. Sure! Just don't play tricks again, got it?" I warned.

"Got it," she replied, "As long as you keep that secret safe."

"My lips are sealed," I replied.

* * *

**Woo! This chapter was exciting, wasn't it? Aw shucks, it wasn't. But this is the second-to-last chapter, so be on the lookout for the last!**

**Also, I want to thank again kevinkev18. Because without you, the idea wouldn't have surged. Thank you!**

**Bye!**

**~Mayu280**


	5. Final Prank

**Chapter 5: Goodbye!**

**So...Marceline's FINALLY leaving. This is the last chapter, so it'll be kinda short. But no worries, there'll be a funny part at the end.**

**Enjoy!**

* * *

_Fionna's POV_

Marshall opened the door of his house as Marceline floated out with her black umbrella in one hand, and her green duffel bag on the other.

"Well," she sighed, "It was nice meeting ya, Fionna."

She leaned close to my brother's ear. "Take care of her, baby bro. This one's a keeper."

Marshall Lee blushed. "Marceline!"

Marceline laughed and looked back to me. "Hahaha, you don't change. Anyway, Fi..." she handed me a holo-message tape. "Remember, do the prank the way I showed you, got it?" she whispered.

I nodded. "Got it. You sure this will work on Cake?"

"Of course," she replied, "It's fool-proof. I've tried it on a couple peeps of mine. Should work on your sister as well. Oh and,"

She smiled and held a finger to her mouth. "And keep that secret quiet, got it?"

I nodded. "Your secret is safe with me."

Marceline switched to a small bat form and began flying away. "Goodbye! See ya guys another time!"

"Goodbye!" Marshall said. "And good riddance," he chuckled.

Once she was out of sight, he turned to face me. "So, you're leaving me too, huh?"

"Yeah, but I'll see ya in two weeks!" I said pecking his cheek. "Bye!"

And I ran my way to the tree fort.

* * *

When I got there, I waved at Cake, who was reading on the couch.

"Well, look who decided to show up!" she laughed. "How was this weekend with Marshall?"

I grinned. "Heh, great. I met his sister from Ooo, you know, where cousins Finn and Jake live?"

Cake's eyes widened. "Really? Cool! Anything else?"

I tossed cake a holo-message tape. "She left you a message. You should play it on the holo-message player."

Cake stretched her arm and reached the device. "Got it. Wait, aren't you saying back?"

I stopped my pace up the stairs. "Uh, naw, I already know what she left you. I need to talk to Beemo and see if he did me a favor."

My sister tilted her head in confusion. "Well...okay."

I rushed up the stairs and counted to myself to see how long it would take Cake to see the prank.

"Five, four," I sat down on the bed, "Three, two, one..."

"EEEEEEEEEEEEEKK! FIONNA!"

I snickered and Beemo entered the room right on cue. "What happened to Cake?" the little videogame station asked.

I sneaked under the bed and laughed. "Marshall's sis left her a prank on holo-message. Hey, did ya download that game I told you?"

Beemo nodded. "Yes. Super Star Car Racing, downloaded succesfully."

I nodded. "Good, now if Cake asks, I'm not here."

Beemo clapped. "Okay!" And he ran off.

I snickered, hearing Cake yell out for me.

"FIONNA! WHAT IS THE MEANING OF THIS? COME DOWN RIGHT NOW!"

I yelled back tauntingly. "Never!"

Laughing, I moved more to the wall under the bed. I'm so glad Marceline thought of this prank.

* * *

**Thanks, my beloved followers, for awaiting this cursed last chapter!**

**I'd like to thank the review by Puckabrinaluver, who suggested Marceline and Fionna did something to freak out Cake.**

**Now, don't ask me what Marceline recorded on that holo-message, all you need to know is that it was one of Marcy's freakishly scary mutations.**

**I thank you for following up to this final point. As of today, this story is COMPLETE.**

**I'd like to thank all the followers of this story for being patient. **

**Meet you all in another story!**

**Goodbye!**


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